1. Field of the Invention
The present invention relates to an absolute encoder for measuring an absolute value of a rotating angle of an object to be measured.
2. Related Background of the Invention
Up until now, a rotary encoder has been employed for angular measurement in machine tools, FA (factory automation) apparatuses, and controlling ends (steering) of automobile, etc. In particular, as an angle detecting system of optical type rotary encoders of the rotary encoders, an incremental type and an absolute type have been known to the public. The absolute type rotary encoder (hereinafter called an “absolute encoder”) generally ensures high accuracy and has an advantage by which errors are not accumulated.
In the absolute encoder, generally, a rotating plate in which light transmitting portions such as slits are provided at a prescribed pattern is attached to the rotation axis. And, a light source and a photodetector are installed with the rotating plate placed therebetween, wherein an absolute value of the rotating angle of a rotation axis is measured by detecting light, which is supplied from the light source and passes through the light transmitting portion of the rotating plate, by means of a photodetector.
Encoders that are disclosed in Document 1 “Japanese Translation of International Application (Kohyo) No. 2000-514199” and Document 2 “Japanese Published Unexamined Patent Application No. 2002-39727” have been known as prior art absolute encoders. For example, in the encoder disclosed in Document 1, a slit consisting of an eddy pattern is provided in the rotating plate, and a rotating angle is measured by detecting light passed through the eddy slit by means of a one-dimensional image sensor.